Gas turbine engines requiring low emissions over normal operating ranges between 100% (“full load”) and part load (e.g. about 70% of full load) can achieve this in three basic ways, all by reducing air mass flow into the combustor in order to maintain an acceptable fuel/air radio without producing excessive poisonous CO gas caused by ultra lean combustion.
First, by use of so called two shaft turbine engines having a gas generator module and a power module each with separate, rotatably independent shafts, the gas generator module is purposefully controlled to have a reduced speed and thereby automatically a reduced air mass flow at part load.
Second, single shaft turbine engines can be configured to dump a fraction of the air mass flow from the compressor overboard, upstream of the combustor, at the expense of overall efficiency, or to bypass the combustors with part of the air mass flow and re-inject it in front of the turbine, thereby conserving the energy of the compressed air.
The third way to reduce air mass flow at part load conditions is to throttle the air going into the compressor by using moveable inlet guide vanes, to direct the inlet air into a swirl in the direction of rotation of the inducer position of a centrifugal compressor or the first stage of an axial compressor.